A five-shot lead does give Leishman some margin for error.
"The game's feeling really good", Leishman told the PGA website.
"My ball-striking was probably close to the best it's ever been right now", he said, "so hopefully I can keep going and make a few more putts". Very easy to throw a mediocre round in there.
Leishman shared the 54-hole lead at the Dell Technologies Championship TPC Boston two weeks ago with Justin Thomas, who surged ahead in the final round to win the event.
Leishman has never led the first three rounds of a PGA Tour event and came from behind during his two PGA Tour wins; the 2012 Travelers Championship and the Arnold Palmer Invitational earlier this year.
"It's definitely one of those courses where if you strike it well and roll some good putts you can go really low, as Jason and Marc have shown over the first two rounds".
50 million affected in Mexico quake : Three days mourning declared
It also triggered a tsunami warning and the evacuation of thousands of people in coastal communities in Chiapas. In September 1985, a magnitude 8.0 quake killed an estimated 9,500 people in and around Mexico City.
Rory McIlroy knows that after his second round at the BMW Championship his slim chances of advancing to East Lake to defend his FedExCup title are as good as over.
Spieth heads the FedExCup standings -- and the race for the $US10 million ($F20m) bonus awarded to the overall season champion -- with only next week's Tour Championship in Atlanta remaining after this week.
The 33-year-old Australian weathered a shaky start with several errant tee shots and a bogey at the fifth but has bagged three birdies to move to 21-under-par through 12 holes at Conway Farms Golf Club.
After a rocky 16 months without a victory, compounded by back problems and motivational issues, Day, 29, decided to make the switch and put one of his old buddies on the bag with just two events to play in the PGA Tour season.
Fellow Australian Day (5/1), Charley Hoffman (16/1) and Jamie Lovemark (25/1) were his nearest challengers on seven under, while Fowler (7/1) lies in the group a further shot back after birdieing six of his final seven holes. Rickie Fowler third at 16 under.
"I guess I just fly under the radar a lot", said Leishman, a playoff loser at the 2015 British Open won by Zach Johnson at St. Andrews. Tomorrow I got to try not to do that.